On Wednesday 28 October 2009, Andi Kleen wrote:> > > However some architectures need special operations on compat pointers> > > (s390 iirc), but if you don't support those it might be reasonable> > > to not support that.> > > > s390 has to sign extend all 32-bit compat process pointers when> > processing them in the 64-bit s390 kernel. I think one other> > architecture has this kind of situation too.> > Which other architure? I reviewed all the definitions in tree> and don't see any other than s390 doing magic there.

I'm also pretty sure that s390 is the only one needing this, Iadded the compat_ptr stuff initially.

Note that a cast from pointer to unsigned long to u64 and backin C does the correct 31 to 64 bit extension, which btw is nota sign-extend but a unsigned extend clearing the upper 33 bits.

The easier rule to remember should be to always to compat_ptr()on any pointer coming from user space, and to avoid pointers indata structures where possible, as DaveM pointed out.